Newly manufactured quantum dots (QDs) typically have very high photo-luminescent quantum yield (PLQY, or simply, “quantum yield”) when measured at low concentrations with the native ligands from the synthesis of the materials still remaining on the surface. However, quantum yield typically decreases when the quantum dots are processed further, for example, into devices such as LEDs, where they may be used as the down converter component. For example, according to one method for fabricating LED packages, quantum dots are formed, and then an insulator layer is added that encapsulates each quantum dot for stability. The encapsulated quantum dots are then incorporated into a polymer slurry, and the slurry is dispensed into LED devices and cured. After curing, the PLQY may be measured. During this process, if there is no “brightening” step, the quantum yield of the quantum dots may decrease by over 50% compared to the quantum yield of the quantum dots prior to their incorporation into the LED devices. The quantum yield, however, substantially recovers during the initial hours of operation of the LED devices. This presents a problem for the customer (e.g., an LED manufacturer) who receives an LED device: the customer needs to operate the LED device for a period of time before the quantum yield reaches target specifications. FIG. 12A is a graph of the change in PLQY of quantum dots as they are brightened by “burning in” the LED device, at different flux densities and temperatures, for time frames up to 10 hours. No other brightening steps were used. Brightening in this way requires a considerable period of time for burn-in prior to use of the LED devices. FIG. 12B is a graph of the change in the red photon output of LED devices incorporating quantum dots (in this case red photons, as the quantum dots used were emitting in the red part of the spectrum) as they are tracked in an integrating sphere. Prior to measuring these devices, they are exposed to various external light flux densities (but no heating), with higher flux densities resulting in larger degrees of brightening, and no heating. This shows that exposure to external light prior to measurement increases the red output and decreases the transient in the red output measurement.
It is appreciated that QD PLQY varies for different conditions and can change over time in the presence of heat and/or light. Other prior art solutions have been proposed involving altering the ligands present at the QD surface as well as sealing the QDs in an optic so that the QDs' local environment is preserved and there is no further shifting of the quantum yield after the optic is made. However, for QDs used as down converters directly on an LED chip, a sealed optic is not possible, and ligand modifications can have a detrimental effect on the long-term device reliability. What is needed is a method for improving QD PLQY without requiring hours of running the LED devices into which they are incorporated.